1. Field of the Invention
The present invention generally relates to a mobile telecommunication system. More particularly, the present invention relates to a hard handover method and a radio network controller therefor in a mobile telecommunication system that can improve the performance of an interfrequency hard handover.
2. Description of the Related Art
In a mobile telecommunication system, handover technology is mandatory to provide seamless service to terminals on the move. A terminal on the move can request a handover, or a network for controlling the terminal can command the handover. When the terminal requests the handover, the network that is currently controlling the terminal newly allocates radio resources and makes the terminal use the newly allocated radio resources. When the network commands the handover for the terminal, the terminal is allowed to use other radio resources according to a need in an operation of the network.
For example, a Wideband Code Division Multiple Access (WCDMA) system serves as a CDMA-based system. In this system, different users receive services using identical frequency resources and different codes. The different users can receive the services because frequencies are different even when different frequency resources and identical codes are used.
For this reason, a service provider provides service at one or more frequencies in a region where a large number of users are located, and provides service at one frequency in a region where a small number of users are located. A user may move to a new service region, which uses only one frequency, from a region where a large number of users are located. In this case, the user must change to a new use frequency in order to receive a seamless service if the frequency of the new service region is different from that of the previous service region.
A conventional handover of the WCDMA system is an intrafrequency soft handover. However, an interfrequency hard handover is performed in the above-described case.
When the soft handover is performed, the terminal does not need to newly acquire synchronization because the terminal reports a timing difference between a handover target cell and the current cell in a call connection state to a system and the system makes the handover target cell ready for the handover using the reported timing difference.
On the other hand, when the hard handover is performed, its probability of success is lower than that of the soft handover. The hard handover requires a more precise handover algorithm than the soft handover.
When performing the interfrequency hard handover, the system makes the terminal measure the strength of a signal received from a cell with a different frequency and report the measured strength. The system sets the interfrequency hard handover using a reported value and commands the interfrequency hard handover for the terminal. At this time, the value reported from the terminal is different from that reported from a cell with the same frequency. A timing difference between the current cell and the cell with the same frequency is correctly reported, but a timing difference between the current cell and the cell with the different frequency is not correctly reported. This is because the terminal in the call connection state can search for the target cell with the different frequency, but cannot obtain correct timing information. Accordingly, when the interfrequency hard handover is performed, the system makes the target cell with the different frequency ready for the handover using timing information different from that used in the existing call connection state. Moreover, the system commands the terminal to newly acquire synchronization while performing the interfrequency hard handover.
This hard handover is referred to as the timing re-initialized hard handover.
The timing difference relative to the target cell with the different frequency is not reported from the terminal when the interfrequency hard handover is performed. For this reason, when the target cell with the different frequency prepares for the handover, arbitrary timing information is basically used for synchronization in the terminal. When receiving an interfrequency hard handover command, the terminal is to newly acquire synchronization while the interfrequency hard handover is performed. Because a new synchronization acquisition is required for the interfrequency hard handover, a handover success probability decreases and therefore a call cut-off time increases when the interfrequency hard handover is performed.
Accordingly, there is a need for an improved method and radio network controller for hard handover in a mobile telecommunication system.